In fast service and carry-out food businesses, it is necessary to package food products in containers which are inexpensive, easy to ship and store, and convenient to use for the retailer and the consumer. When assembled and filled with food products, such containers must be easy to close and open, and yet they must remain shut in transport from the food counter to a table or other carry out location. It is also important, in the design and construction of such containers, to take into account environmental aspects of their use.
One form of packaging which has addressed these concerns, and has met with acceptance, is a carton comprised of a joined tray and cover with integral latching means which can be readily formed from a cut and scored blank. The cut and scored blank is made from paperboard, boxboard or corrugated board. When partially folded and glued, the blank forms a hingedly connected tray and cover, each in the form of a truncated pyramid. Such partially folded cartons can be shipped and stored in nested condition. In use, various food products, such as hamburgers, chicken pieces, pizza slices and the like, are placed in the tray portion of the partially folded carton, and the cover portion is folded over the tray portion and latched thereto at a location opposite the fold or hinge in the back panel of the carton. Such double cavity, folded tray and cover, cartons are referred to a clamshell cartons. A clamshell carton is disclosed, for example, in U.S. Pat. No. 4,877,178, issued Oct. 31, 1989 to Dopaco, Inc. as assignee.
One problem which has been encountered with such clamshell cartons, particularly when they are made from thinner grades of paperboard, has been their tendency to buckle in the back as they are being closed by restaurant crew persons or opened by customers. For example, when the top cover portion is lifted and folded back on the center hinge in the rear portion of the carton, the region adjacent the hinge tends to flex and bow. This problem is sometimes referred to as "oil canning" because the back region of the carton can distort as opening begins and then pop back into its proper and intended configuration, in much the same way as an oil can lid pops back into position after being depressed. The buckling problem appears to be caused by the flexible nature of the carton material, the spacing between the front of the carton and the back region where the buckling occurs and the forces applied to the carton portions as they are handled during the closing and opening operations. This problem is a troublesome and awkward one for crew persons trying to rapidly assemble and fill nested and partially folded blanks, and for consumers trying to open the carton and access its contents. Sometimes, a consumer can alleviate the problem during opening by pressing his fingers in the buckled region to realign the carton portions. In other instances, however, trying to fully open the cover and forcibly overcome the buckling can result in a torn container and/or spilled contents.
The prior art has addressed this buckling problem. U.S. Pat. No. 4,232,816 discloses the use of one or more slits which are normal to, and bisect, the fold line in the rear wall of a clamshell container. Such slits are said to weaken the center portion of the rear wall and thereby reduce the tendency of that wall to buckle upon opening. U.S. Pat. No. 4,266,713 discloses the provision of lines of weakness so positioned relative to the center hinge as to create a separate hinge ridge or an inverted hinge channel upon closing the carton. Such a hinge ridge or hinge channel is said to stabilize the rear carton wall against buckling and/or bowing outwardly. U.S. Pat. No. 4,792,085 discloses various diagonal, intersecting incisions above and below the hinge line of a clamshell container, as well as ribs formed above, below and parallel to the hinge line.
These efforts of the prior art, while directed at the buckling problem associated with clamshell cartons, have not been widely accepted; and have been found to be less than entirely satisfactory.
Accordingly, it is a primary object of this invention to provide a novel clamshell carton construction which can be easily made and conveniently closed or opened without encountering undesirable buckling or "oil canning" effects.